Why Was the Ottawa Earthquake Felt So Widely?

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The moderate magnitude-4.4 earthquake that rattled Canada and the
Northeast this morning (May 17) made a big impact thanks to old
bedrock.

Quakes on the East Coast are generally more widely felt than out
West because of differences in the Earth's crust between the two
regions, according to the U.S. Geological Survey (USGS). East of
the Rockies, earthquakes
of the same size are often reported as felt across an area 10
times larger than in the West.

The East Coast has been a pretty quiet place, geologically, since
the Atlantic Ocean opened 160 million years ago. Compared to the
West Coast, the Earth's crust out East is older, denser and more
uniform. That helps earthquake waves zip through the crust.

Along the Pacific, a conveyor belt called a subduction zone (a
collision between two tectonic plates), as well as the
strike-slip San Andreas Fault, have plastered hundreds of miles
of new crust onto North America. These active plate boundaries
created a mishmash of rocks and faults that hinder earthquake
waves.

Today's earthquake hit in the Western Quebec Seismic Zone, the
site of a 2010 magnitude-5.0 earthquake felt as far away as Ohio,
the USGS said. (A magnitude-5.0 earthquake releases eight times
more energy than a magnitude-4.4 earthquake.) Damaging
earthquakes occur in the zone about once a decade, while a
smaller earthquake strikes three to four times a year, according
to the USGS.

The biggest historic earthquake in the Western
Quebec Seismic Zone was an estimated magnitude-6.2 in 1935
that toppled chimneys in Northeastern Ontario.